1) WHY IS TERUMAH NOT A "DAVAR HA'NADUR"
QUESTION: The Gemara teaches that if a person makes a Neder by comparing an object to Terumah, the Neder is not valid because Terumah is a "Davar ha'Asur" and not a "Davar ha'Nadur."
Why is Terumah not a Davar ha'Nadur? Terumah is a form of Hekdesh; the owner of the fruit separates a portion of his fruit and declares it to be Terumah, rendering the fruit forbidden to a Yisrael. Why is it not considered a Davar ha'Nadur like Hekdesh?
(a) The RAN explains that if it is the owner's declaration which makes the fruit forbidden as Terumah, the Terumah should be forbidden to everyone. The fact that Terumah is forbidden only to Yisraelim and not to Kohanim shows that it is not the person who creates the Isur, but the Torah. The person merely separates the fruits and calls them "Terumah," and then the Torah makes them forbidden to Yisraelim and permitted to Kohanim. In contrast, when a person sanctifies an object as Hekdesh, it is he who creates the Isur and makes the object forbidden to everyone in the world.
(b) TOSFOS and the ROSH explain that even before the fruits of Terumah are separated, the fruits are forbidden as Tevel. When one separates the fruits and declares them Terumah, he actually makes them permitted and not prohibited, since they become permitted to Kohanim. Accordingly, Terumah is not a Davar ha'Nadur, because the person does not create an Isur when he separates Terumah.
RAV YOSEF ENGEL (in ASVUN D'ORAISA) explains that the argument between the Ran and Tosfos may depend on the nature of the Isur of Tevel. Tosfos maintains that the Tevel is forbidden is because of the Terumah it contains. The Ran, on the other hand, maintains that Tevel is an independent Isur which is unrelated to the Isur of Terumah. Although the Isur of Tevel is removed from the fruits when one separates some of the fruits and calls them "Terumah," the Tevel fruits are forbidden not because of the Terumah they contain but because of the independent Isur of Tevel.
According to Tosfos, why is Tevel forbidden to Kohanim? If the Isur of Tevel is merely due to the Terumah mixed with the fruits, Tevel should be permitted to Kohanim since Terumah is permitted to them. The answer is that according to Tosfos, Terumah is also forbidden to Kohanim since it belongs to Hash-m. The Torah permits them to eat it only when it has been separated from the rest of the fruits, because "mi'Shulchan Gavoha Ka Zachu" (see TOSFOS to Shabbos 24b, DH l'Fi). The Ran perhaps disagrees with Tosfos because he maintains that "mi'Shulchan Gavoha Ka Zachu" applies only to Kodshim and not to Terumah (as the RI indeed states, as cited by Tosfos in Shabbos there).
(c) The RAMBAM explains that although a person chooses which fruits to make Terumah, he cannot create an Isur of Terumah on fruits which have no obligation of Terumah. Since the status of Terumah comes about only through an obligation, it is called a Davar ha'Asur and not a Davar ha'Nadur.
However, if an Isur which comes about only as a result of an obligation is a Davar ha'Asur, why may a person make a Neder by comparing an object to a Korban Chatas (13a)? Such a Korban also comes about through an obligation, and thus it should be considered a Davar ha'Asur and not a Davar ha'Nadur!
The Rambam answers that there is one type of Chatas which is brought voluntarily -- the Chatas of Nazir (a person voluntarily makes himself a Nazir and then brings a Chatas when his Nezirus concludes). Since all Korbenos Chatas are in the same category of Korban, a person may make a Neder even by comparing an object to an ordinary Chatas (a Chatas Chelev), since there exists a type of Chatas which is brought voluntarily.
2) "HATFASAH" TO A "BECHOR"
QUESTION: Rami bar Chama asks (11b) whether one's Neder takes effect when he compares (with Hatfasah) the object to the meat of a Korban Shelamim which rests in front of him after the blood of the Korban has been sprinkled on the Mizbe'ach (l'Achar Zerikas Damim) so that the meat itself is Mutar. What is the person's intention? Does he intend to compare the object to the Korban in its original state ("b'Ikro Ka Matfis"), before the sprinkling of its blood, when it was Asur, in which case the Neder does not take effect because he compares the object to a Davar ha'Asur? Alternatively, does he intend to compare the object to the Korban in its present state, "b'Hetera Ka Matfis," after the sprinkling of its blood, when it is Mutar, in which case the Neder takes effect because he compares the object to a Davar ha'Nadur?
The Gemara here (12b) cites a Beraisa in an attempt to prove that this question is the subject of a Machlokes Tana'im. The Beraisa discusses a case in which a person makes a Neder by comparing the object to a Bechor. Rebbi Yakov prohibits the item (the Neder takes effect), and Rebbi Yosi permits it (the Neder does not take effect). The Gemara suggests that the Beraisa refers to one who makes a Neder by comparing the object to the flesh of a Bechor after Zerikas Damim, and the dispute between Rebbi Yakov and Rebbi Yosi is whether "b'Ikro Ka Matfis" or "b'Hetera Ka Matfis."
The Gemara refutes this explanation of the Beraisa and gives a different explanation for the Beraisa's case. The Gemara explains that Hatfasah to a Bechor differs from Hatfasah to an ordinary Korban because a Bechor is sanctified from birth (Kadosh m'Rechem) and does not need to be sanctified by the owner. Hence, Rebbi Yosi maintains that it is a Davar ha'Asur and the Neder does not take effect. Rebbi Yakov maintains that although it is Kadosh m'Rechem, the owner still has a Mitzvah to consecrate it, and thus it is considered a Davar ha'Nadur and the Neder takes effect.
Why does the Gemara initially suggest that the dispute in the Beraisa is whether a person is Matfis "b'Ikro" or "b'Hetera"? If this would have been the subject of the dispute, both Tana'im would have expressed their views in the more general case of one who makes a Neder by comparing the object to the meat of a Korban Shelamim after Zerikas Damim!
ANSWER: Perhaps the Gemara initially assumes that the Tana'im discuss a case of Hatfasah to a Bechor because a Bechor may not be eaten by non-Kohanim (according to Beis Hillel in Bechoros 32b). One may have thought that since the meat of a Bechor is forbidden to non-Kohanim, it is a Davar ha'Nadur and a person may make a Neder by being Matfis to it, even according to the opinion that a person is Matfis "b'Hetera." The Beraisa therefore teaches that Rebbi Yosi says that one cannot be Matfis to it just as one cannot be Matfis to Terumah which is also forbidden to non-Kohanim, since it is Mutar to Kohanim (see previous Insight).